The closing of the American frontier is dated at 1890, which also marks the beginnings of American Imperial expansion.Mostly in the Pacific, and the Caribbean (seizure of Spanish overseas territories, during the Spanish-American War). Eventually, the Americans learned that if you wanted the resources of other countries, and you wished to open them up as markets for American goods, you establish puppet regimes, and prop them up with the US military, if need be. It was much cheaper than permanently garrisoning them with troops, as Britain, and various European powers had tried. But with the creation of the Soviet Union, and the Peoples Republic of China, US military intervention, augmented with clandestine intelligence operations became more frequent. This, to protect US Imperial and Strategic interests from the bogeyman of 'Communism'. The communist bogeyman has now been replaced with the bogeyman of Islam. The US is still an imperialist country, with troops in over 100 other countries, worldwide. Has it reached its 'twilight of empire'? The future will tell.

An empire uses its military power to exploit other countries economically. The Roman Empire and the British Empire are the best examples. The USA is more of a world policeman. We're focused on maintaining order worldwide at a great cost to US taxpayers, and we are not benefiting economically.